This invention relates generally to the control of a high-speed, solid-state, unidirectional electric valve of the kind that will switch from a non-conducting (off) state to a conducting state (on) in response to a relatively positive control voltage being applied to a gate electrode of the valve and that will subsequently turn off in response to a negative control voltage being alternatively applied to the same gate, and it relates more particularly to means for detecting and responding to a malfunctioning valve that fails to turn off when negative control voltage is applied to its gate.
A valve of the kind described is known generally as a gate turnoff (GTO) thyristor. It is a multi-layer semiconductor designed to freely conduct "forward" anode current (i.e., current flowing into its anode and out of its cathode) when its gate electrode is triggered by a suitable turn-on or firing signal. A GTO thyristor is distinguished from a conventional thyristor by its ability to interrupt or block forward anode current if a voltage of relatively negative polarity and appropriate magnitude and duration is applied across its gate-cathode junction. Such voltage is negative in the sense that the electrical potential of the gate is negative with respect to the cathode. It causes current to flow in a reverse direction in the thyristor's gate. In other words, to turn off a GTO thyristor current is drained from the gate. Hereinafter such current is referred to as either "negative gate current" or the "turnoff signal."
In normal operation, the anode current-blocking or turnoff process of a GTO thyristor can be initiated at any time without waiting for a natural or externally forced zero crossing of the anode current. During the turnoff process the negative gate current rapidly rises to a high peak that depends on the magnitude of anode current to be interrupted and then subsides as the thyristor recovers its ability to withstand off-state anode voltage. Once a turnoff process is successfully completed, the resistance of the gate-cathode junction is very high and limits negative gate current to a trivial magnitude.
Occasionally a GTO thyristor malfunctions, failing to turn off after the turnoff signal is applied to its gate. Typically the failure mode is an internal short circuit between the anode and the cathode of the thyristor, and this normally results in a shorted gate. Means for detecting such a failure would be useful for both diagnostic and protective purposes. When a shorted gate occurs, the gate-cathode resistance of the thyristor remains low and will not limit gate current effectively. Under this abnormal condition an undesirably high magnitude of negative gate current would continue to flow so long as the negative control voltage is applied to the gate, and the large amount of energy drained from the failed GTO thyristor could seriously overheat or otherwise damage the associated control circuit.